1. Field of the Invention
The present invention relates to a roaster with a stabilized fluidized bed for roasting zinc concentrate, and more particularly to a fluidized bed roaster for zinc concentrate which forms a stabilized fluidized bed to reduce the content of sulfide sulfur, produced due to incomplete desulfurization of zinc concentrate, in calcine produced by the roasting of zinc concentrate.
2. Description of the Related Art
As well known to those skilled in the art, the so-called roasting is a process for converting metal ores to their oxides by heating the ores at a temperature below their melting point for the purpose of easily reducing the ores in a subsequent process. For example, in the case of iron ore, magnetite is roasted to generate hematite. Zinc concentrate (ZnS) is roasted to oxidize its sulfur component into sulfur dioxide, thereby producing zinc calcine (ZnO).
There are various types of roasters, which are furnaces used for the roasting process, and the roasters include, for example concerning zinc, a multiple hearth roaster, a flash roaster, a fluidized bed roaster, etc.
Although brands such as MacDougall Furnace, Herreshoff, Wedge, Skinner, etc. supply multiple hearth roasters, they are limited only to small-scale sulfuric acid factories but are not used in large-scale roasting process such as in a refinery. In the flash roaster, during the falling of ore particles dried by heating, most of combustible components of the ore particles are instantaneously oxidized. Accordingly, the ores have to contain sufficient combustible components to maintain the required temperature, and their particles should be sufficiently fine to be calcined thoroughly within the falling time of the particles. Owing to short contact time of ore particles with roasting gas, there are disadvantages in that it is required that ore particles should be milled sufficiently into fine particles and that the temperature of the roasting chamber should be maintained at a high enough temperature to finish the roasting within such a short contact time.
Fluidized bed roasters, to which the present invention relates are designed to allow ore particles and roasting gas to come into contact with each other for a sufficiently long time. Such fluidized bed roasters include a dry type and a wet type. In the dry type, dry ores containing about 10% moisture are fluidized to be roasted. In the wet type, wet ores (slurry) containing approximately 25% moisture are poured to be roasted.
As the dry type roaster for roasting zinc concentrate, there is LURGI-VM T.M, which was put to practical use in the latter half of the 1950""s, and became popularized for commercial use between the 1960""s and the 1970""s. The roasting process in such a roaster involves the steps of blowing hot air into the roaster to increase the internal temperature of the roaster to a required temperature for roasting concentrate, supplying oxygen required for roasting zinc concentrate through an air supply port 4-1, while charging zinc concentrate through a concentrate charge port (not shown) arranged at one side of a lower cylindrical section 3-1, by which the zinc concentrate is exposed to oxidizing atmosphere to undergo an oxidation process. Because the oxidation is an exothermic reaction, further feeding of hot air is unnecessary to maintain the internal temperature of the roaster.
It is only necessary to regulate the amount of charged concentrate so as to control the internal temperature of the roaster. Zinc calcine (ZnO) oxidized from zinc concentrate (ZnS) by the roasting treatment is discharged through a calcine discharge port 8-1 and collected, which port is arranged at the other side of the lower cylindrical section 3-1 above the concentrate charge port.
Roasters used in the above-described process to roast zinc concentrate have been developed to be applicable to a specific zinc ore roasting method improvably modified from a conventional sulfide iron ore roasting method to meet the dissolution and electrolysis characteristics of zinc. A recent tendency is to pulverize zinc concentrate into fine particles at a mine to improve the grade and the recovery rate of zinc, in pace with improvements in mining technology. Such fine concentrate particles have a short residence time in conventional roasters for roasting zinc concentrate. Thus, unburned sulfide sulfur (hereinafter referred to as S.S.) increases in quantity. Also, the angle of repose, 7-1, of an intermediate tapered section 2-1 in the conventional roaster of FIG. 2 or 3 is limited to 20xc2x0, by which the space velocity of roasting gas is faster than the flow rate of the particles. As a result, a great quantity of roasted calcine is scattered around and carried over to the gas discharge port of the roaster, along with the gas. The carried-over calcine may be attached to the wall surface and tubes of a boiler, which is downstream processing equipment, thereby resulting in accelerated pressure loss and a contamination of gas purifying facilities. Therefore, it is difficult to achieve a continuous operation. Thus, there is a problem in that the yearly output and real yield of zinc decline.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a fluidized bed roaster with a stabilized fluidized bed for roasting zinc concentrate, which can have a uniform internal temperature distribution to form a stabilized fluidized bed, thereby dramatically reducing the amount of sulfide sulfur contained in calcine and the amount of fine calcine particles carried over to a gas discharge port, while achieving an increase in productivity.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a fluidized bed roaster for roasting zinc concentrate, comprising: an upper cylindrical section closed at an upper end thereof by a top circular roof fixedly mounted to the upper end, the upper cylindrical section having a volume corresponding to 3.8 to 4.8 times the volume of a lower cylindrical section; an intermediate tapered section fixedly coupled, at an upper end thereof, to a lower end of the upper cylindrical section, the intermediate tapered section having a structure downwardly tapered at a repose angle of 22 to 25xc2x0 while having a volume corresponding to 1.7 to 2.2 times the volume of the lower cylindrical section; the lower cylindrical section fixedly coupled, at an upper end thereof, to a lower end of the intermediate tapered section; and an air supply port coupled to a lower end of the lower cylindrical section, the air supply port having tuyeres arranged in a density of 110 to 135 tuyeres per unit area (m2).
Preferably, the tuyeres have a diameter of 5 to 5.8 mm and a pitch of 85 to 95 mm.